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Old 05-29-2014, 08:38 AM   #1
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PT41 Vibration and Noise Abatement

Hi All:

I'm the proud new owner of a 1984 Cheermen PT41 with twin Lehman 120's.

When I bought her I (Stupidly) spent all my time on the flybridge during the sea trials. Mechanics and surveyor were below checking everything out which came back A-OK. Great!

BUT now that I've spent some time down below I really notice how unbearably noisy it is and how much vibration there is underway.

Any suggestions? She has new motor mounts in the last 3 years, could they be too hard a not isolating; the whole boat shake and vibrates at lower rpm.

Would an intake silencer help? Does anybody make those?

Engine room soundproofing? IS it worth the money? How much does it help.

Any and all thought GREATLY appreciated by myself and MY WIFE!

Thanks
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Old 05-29-2014, 10:10 AM   #2
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Noise and engine/drive train vibration are two different things- different causes and different solutions.

First tackle the vibration. Three year old engine mounts sounds good. But what about prop shaft alignment and prop condition. Aligning the shaft isn't difficult- do it your self or have the yard do it for a couple of hundred$. The prop may be dinged, bent, not seated properly on its taper, etc. The only way to know is to haul the boat, inspect the props and send them out for trueing up if necessary. Did the surveyor at least measure the blade to blade trueness? They should all line up within a 16th of an inch or so.

Noise is tougher. Look for sound paths like intake vents. Maybe baffling them could help. Full Soundown insulation can also help, but it is expensive. How much is anyone's guess.

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Old 05-29-2014, 10:16 AM   #3
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When we took Moonstruck on a sea trial, I would not drive, but stood in the center of the cockpit. I told them what speeds to do. When we got back, I told them I wanted the shafts checked for alignment, new cutlass bearings, and the props sent out for checking. When it went back together, it was smooth as silk.

Lehmans are not known for smooth idling. I don't know if softer mounts would help much. An engine synchronizer could help at speed. Good engine room insulation and sealing hatches should help on noise. These folks make some really good sound attenuation material that we have on Moonstruck.

Soundown (Sound Down, sound down, sounddown) - The Woldwide Leader in Noise Control Engineering

Sealing bulkhead cut outs and hatches will help a lot. I would take baby steps to see what works on your boat.
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Old 05-29-2014, 12:06 PM   #4
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In additon to checking the props, shafts and alignement as suggested above you might have a good look taken at the engine mounts.

One of them could be misadjusted, not faulty, but it's called 'soft foot' meaning one is not carrying its fair share of the load. That can increase shaking particularily at low speed where the engine is rougher running to start with.

Mounts may take a bit of an initial set after installation.

Sometime one of the adjustment nuts comes loose so get that checked.
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Old 05-29-2014, 01:26 PM   #5
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Thanks Moonstruck a lot of good thoughts there.

I wonder though, how to "Sealing bulkhead cut outs and hatches" when the floor is 4" thick and still needs to be able to be opened easily?
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Old 05-29-2014, 01:28 PM   #6
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Quote:
Originally Posted by C lectric View Post
In additon to checking the props, shafts and alignement as suggested above you might have a good look taken at the engine mounts.

One of them could be misadjusted, not faulty, but it's called 'soft foot' meaning one is not carrying its fair share of the load. That can increase shaking particularily at low speed where the engine is rougher running to start with.

Mounts may take a bit of an initial set after installation.

Sometime one of the adjustment nuts comes loose so get that checked.
I'm going to have the local mechanic check that out next week as well, thanks C lectric
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Old 05-29-2014, 05:21 PM   #7
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Originally Posted by Rduval View Post
Thanks Moonstruck a lot of good thoughts there.

I wonder though, how to "Sealing bulkhead cut outs and hatches" when the floor is 4" thick and still needs to be able to be opened easily?
Any kind of thin compression strip could help stop noise from a hatch. It shouldn't make it any less accessible. Expandable foam can be used around bulkhead penetrations.
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Old 05-29-2014, 08:23 PM   #8
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The thing I found that helped the most to make my boat quieter and much smoother from a vibration stand point was to change the engine mounts to soft ones while adding flex couplings between the transmissions and shafts.

That also eliminated all the idle vibration that you find in GBs and other trawlers with Lehmans.
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Old 05-29-2014, 08:30 PM   #9
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Make sure you are not hearing engines that are not properly sync'ed. My new to me Gulfstar has old 4 cylinders that are known to vibrate at low engine rpms, the previous owner had installed soft engine mounts as well as rubber shaft couplers in addition they had about 3" of insulation added to the raised lower station and salon floor. I was down below for a good portion of the sea trial.
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Old 05-31-2014, 08:30 AM   #10
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Originally Posted by Capt.Bill11 View Post
The thing I found that helped the most to make my boat quieter and much smoother from a vibration stand point was to change the engine mounts to soft ones while adding flex couplings between the transmissions and shafts.

That also eliminated all the idle vibration that you find in GBs and other trawlers with Lehmans.
Hi Capt.Bill11, I've heard a bit about soft engine mounts and flex couplings (but admittedly not much). Is there a lot of history on these (Aquaflex are they called).

I have read about alignment being so very critical on the engine to shaft and that soft motor mounts allow the engine/shaft to go out of alignment as rpm (and therefore torque load) change and damaging the cutlass bearings.

Soft mounts + a flex coupliing sounds like a great idea to me, in theory. Are a lot of people using these and is there a reasonably long history of successful use?
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Old 05-31-2014, 10:18 AM   #11
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Rduval,
They are not cheap and one can do w/o them but IMO they are the ideal drive system. Like synthetic oil .. there is no downside except money lost. But unlike synthetic oil there are great benefits to inboard boats.

"I have read about alignment being so very critical on the engine to shaft and that soft motor mounts allow the engine/shaft to go out of alignment as rpm (and therefore torque load) change and damaging the cutlass bearings."
Depends on a lot of variables but basically not so (to an objectionable degree) for most boats IMO.

But w the Aqua Drive the shaft stays centered and is perfectly aligned by the thrust bearing that is installed during the installation of the AD. The installation of these drives is not really easy. I've done it but it was a big job for me. One must construct a mount for the thrust bearing that takes 100% of the thrust of the propeller. It needs to be accurately located and fixed to a very stout part of the hull. The fine adjustment of the position of the thrust bearing permanently aligns the propeller shaft. The PS dos'nt move fwd or aft nor up, down or sideways once the thrust bearing is installed. The engine is coupled to the PS via a CV joint like on a FWD car. The engine then is free to move in any direction within the limits of the CV joint. This is why the soft mounts can be employed. Since engine mounts no longer need to deal w the thrust of the propeller engine mount design need not be compromised .. as is usually the case. Much more of the engine vibration is absorbed by the soft mounts and the PS is not free to jump around causing other vibration issues and wearing the stern bearing slightly faster.

Their history is not 100% good (but nearly so) so much so that if I had lots of money I'd install one on any IB boat I would own. It's the ideal way of doing it.
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Old 05-31-2014, 07:29 PM   #12
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The installation of these drives is not really easy. I've done it but it was a big job for me. One must construct a mount for the thrust bearing that takes 100% of the thrust of the propeller. It needs to be accurately located and fixed to a very stout part of the hull.
Wow, lots to consider. The support you discuss is inside the vessel, behind the transmission? If so a re we talking something fabricated across the stringers that could, for instance, be attached to the motor mount flanges that straddle the stringers?
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Old 05-31-2014, 07:41 PM   #13
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I would try and get some rides on other boats and see what they have and how your boat compares.

I think basic soundproofing is always a nice idea...everything else may or may not have the desired advantage.

My Lehman shakes, rattles and rolls at 650rpm but smoothes out to barely be noticeable above 1000rpm..and with decent soundproofing and a engineroom overhead, airspace , then to cabin deck.... there is enough sound reduction that at 1650 cruise, normal conversation is very easy.

You can spend a lot of money and energy and get little or no results before jumping in with 2 feet...check around to see what you really have and what really might help.
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Old 05-31-2014, 08:33 PM   #14
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Scott's got a good point.
You could install an Aqua Drive and not experience much of any change in seat of the pants feel or noise reduction.
It depends on what's causing the vibration and noise. For example the exhaust system could be shaking a bulkhead and the Aqua Drive probably wouldn't change that.
I did my own installation and had the smallest Aqua Drive. I think it was $1500. If you had your favorite yard install two for your twin engine trawler it may cost $10000.

Also there is Python Drive and other drive systems available now. Aqua Drive had a patent until (very roughly) 15 years ago. I bought mine while their patent was in force.


Rduval,
Yes. Usually a steel frame is fabricated to fit between the engine beds about 12-14" behind the aft end of the transmission. A combination of metal angles and marine plywood all bolted together or a FG layup. Design your own or ask your favorite yard how much will it cost. Then you may rush back to your own ideas.

Look up your nearest Aqua Drive dealer if your'e interested. Aqua Drive has very good customer support. My AD guy never got tired of my questions or ideas. But as Scott suggests you may not need one ... but basically nobody does. You may experience a magic reduction in noise and vibration. I don't have one in Willy. My funds are limited and I have MANY other projects more important. And Willy is quite smooth for a 4 cylinder diesel. Sure I'd rather it was as smooth as an electric motor but I'm not unhappy w the slight vibration that exists.

But Aqua Drive is the ideal way to drive your boat and if you have a gas powered boat it could turn out to be "as smooth as an electric motor". But remember it could happen that you won't notice any difference. Not likely though.
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Old 05-31-2014, 09:19 PM   #15
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Our Pt 38 Sedan has no unbearable vibrations at 1000 RPM or higher, galley stove rattles a little at less than 1000 RPM, so I would suggest checking the alignment with someone who knows how to do it. In regard to the noise like I said in my PM we have carpet with foam underlay which we find very quieting but it still is a diesel underfoot.

Great advice to go out on other boats to see what they are like, maybe the standards you are setting are a little high......
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Old 05-31-2014, 09:24 PM   #16
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My Lehman rattles a bit when in neutral at idle up to around 900 rpm...once in gear and over 900 she settles right down.

maybe I missed it but are your concerns of noise/vibes in or out of gear or both?

If out of gear in neutral...all the aligning, shaft savers, flexi-doodles, etc aren't going to help that at all...

There are a few things that will but make sure you aren't trying to fix something that really isn't an issue but if still annoying...you head off in the proper direction.
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Old 06-01-2014, 08:55 AM   #17
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I'm going to call Aquadrive tomorrow to get some more info. Based on their online pricing their gear will be about $1500 per engine. I'm a retired machinist and I'm sure I can design up a solid mount for the thrust bearing plate and get it made through friends still in the business so I'm figuring less than $6k all in which, if it works as advertised, I consider a bargain. (Much cheaper than the divorce if I don't get the noise and vibration under control)

Can someone tell me how the flanges are attached to the shafts? I'm obviously going to need to shorten them and I'm wondering if they are keyed or have flats machined, etc and have to go to a machine shop or if they're just set screws, etc and, with a lot of patience, a sawzall, 50 blades and cutting fluid; I can shorten them in the boat?
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Old 06-01-2014, 08:57 AM   #18
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Our Pt 38 Sedan has no unbearable vibrations at 1000 RPM or higher, galley stove rattles a little at less than 1000 RPM, so I would suggest checking the alignment with someone who knows how to do it. In regard to the noise like I said in my PM we have carpet with foam underlay which we find very quieting but it still is a diesel underfoot.

Great advice to go out on other boats to see what they are like, maybe the standards you are setting are a little high......
You may be right on all counts, esp the standards but I have to agree with the wife, right now, it's pretty unbearable.
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Old 06-01-2014, 11:40 AM   #19
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Alignment problems rarely cause vibrations. If far off, it can break things, but usually unnoticed til then. Aquadrive type drive shafts/bearings also rarely make much of a change.

Under what conditions do you get the noise/vibration? At dead idle in neutral? In gear at idle? At cruise of 1600?

You need to figure out what is causing the issue before proposing a fix.
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Old 06-01-2014, 12:55 PM   #20
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You need to figure out what is causing the issue before proposing a fix.

Exactly

The only money you should spend on this issue is for a diagnosis if you cannot do it yourself. The best any of us can offer is ideas or areas to look, until diagnosed anything "might" work.
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